Mobile and/or portable radio transceiving devices have heretofore incorporated a "channel guard" coding option which permits an operator to selectively call desired parties by transmitting a low frequency tone or digital data pattern. Such a "channel guard" coding option is also referred to as a continuous tone coded squelch system (CTCSS) or a continuous digital coded squelch system (CDCSS). Channel guard provides a means of restricting calls to specific radios. Only the desired parties' receivers are programmed to decode the transmitted tone or digital data pattern.
By using the channel guard option, many users can share a repeater system, with only the receivers programmed to receive the particular transmitted channel guard coded tone being enabled to receive a transmitted message. The transmitted tones in a tone channel guard system may, for example, range from 67 Hz to 210.7 Hz in 0.1 Hz steps. In a digital channel guard system, there may be over 80 standard digital codes. The frequency of transmitted digital channel guard signals is typically significantly lower than in tone channel guard systems.
The tones or digital codes frequency modulate the transmitted RF carrier at typically .+-.0.75 kHz deviation. The specification tolerance is 0.5 to 1.0 kHz deviation. Voice signals frequency modulate the carrier at .+-.3.75 kHz deviation. Therefore, the total channel deviation is 3.75 plus 0.75 or 4.5 kHz deviation which is within the FCC's 5 kHz maximum limit.
Systems incorporating such tone or data channel guard features include tone or data processing circuitry for processing and decoding the received tones or digital data to detect the proper tone or data sequence. To process such low frequency tones or digital data to correctly detect predetermined tone or data patterns, it is necessary for the RC time constant in the tone processing circuitry to be long enough to preserve the tone or data signal pattern for reliable decoding of noisy signals. In a digital channel guard system, a digital pattern may consist of a lengthy string of logic "1's" or "0's".
In conventional radios incorporating such a channel guard feature, when a receiver detects a carrier in the channel guard mode, it attempts to decode a channel guard tone pattern. In such prior art radios, if the proper channel guard is not detected, the receiver audio is simply muted.
The time required to decode a channel guard tone pattern was not heretofore considered an important limitation in previous designs of tone/data processing circuits. In this regard, a hundred millisecond or more settling delay was not viewed as presenting any significant operating problems.
The present invention, by reducing this settling delay to a minimum, permits scanning multiple channel frequencies to search for the correct channel guard tones or data. In the channel guard operating mode, when a RF carrier is detected, the tone or data processing circuitry of the present invention, rapidly begins to produce tones and digital data which may be reliably decoded by the transceiver microprocessor. If an "incorrect" channel guard pattern is detected, the channel scanning process continues immediately. The present invention allows low frequency data to be sensed as fast as possible while scanning different channel frequencies.
In simultaneously implementing both channel scanning and channel guard features, the present invention recognizes that a problem inhibiting fast scanning of multiple channel frequencies results from the failure of a transmitter to be precisely on frequency with the receiver. In this regard, it is noted that the output of a FM detector with no RF carrier present, consists of noise at an average DC level. If the transmitter is on frequency with the receiver, when an on frequency carrier is detected, the DC level at the output of the FM detector will not shift. Thus, the DC level at the output of the FM detector remains relatively constant when an on frequency carrier appears.
However, if the receiver and transmitter differ by, for example, 0.5 kilohertz in frequency, the DC level at the output of the FM detector will shift. This shift in DC level creates a pulse which is detected by the tone or data processing circuitry which will be misinterpreted as valid data by the processing circuitry until the RC networks therein charge to the new DC level. Since under such circumstances it may take the RC networks in the tone processing circuitry approximately 100 milliseconds to charge to the new DC level, data detected during this "settling" time period may be erroneously interpreted as a part of a tone or data channel guard pattern.
If a receiver and transmitter differ by, for example, 2 kilohertz or more, even longer settling times are required before the tone or data processing circuitry will, in fact, produce valid, reliable channel guard data. Since tone signals or digital data typically frequency modulate an RF carrier at a minimum of plus or minus 0.5 kHz deviation, such a deviation creates a significant impediment to implementing the rapid scanning of multiple channel frequencies while employing the channel guard feature described above.
In an exemplary embodiment of the present invention, a capacitor at the output of the FM detector in the tone/data processing path is charged as rapidly as possible to a new DC level caused, for example, by an off frequency transmitter. In the present invention, a pulse is generated by, for example, the transceiver microprocessor which initiates the rapid charging of the capacitor as soon as it is detected that data needs to be read from the tone/digital data processing circuitry for decoding. Such a pulse may be initiated by the microprocessor whenever channel frequencies are changed and/or the carrier activity sensor becomes active.